Various types of position location systems are known in the art and may be used in a wide variety of circumstances wherein it is desired to locate or determine the position of one object with respect to another object. One type of position location system is radio-based and uses radio signals to determine the relative position of a first object (i.e., having a radio receiver associated therewith) with respect to a second object (i.e., having a radio transmitter associated therewith). Many such radio-based position location systems use the transit time or “time-of-flight” of a radio signal traveling between the transmitter and receiver to determine the distance between the transmitter and receiver. If two or more transmitters are used, triangulation methods may be used to determine more precisely the relative location of the receiver with respect to the transmitters. An example of a radio-based time-of-flight position location system is shown and described in U.S. Pat. No. 6,665,333, issued Dec. 16, 2003, and entitled “Methods and Apparatus for Determining the Time of Arrival of a Signal” which is specifically incorporated herein by reference for all that it discloses.
While such time-of-flight and/or triangulation radio-based position location systems have been used in a wide range of applications and environments, they have proven to be difficult to implement in certain applications and environments. For example, it has proven difficult to implement such systems in “high reflectivity” environments that contain objects or geologic features that reflect the radio signals produced by the transmitters. More specifically, the radio-reflective objects or features create substantial multi-path and other types of interference that can make it difficult, and in some cases impossible, to accurately locate the relative positions of the objects by means of time-of-flight. Still other applications may require the ability to detect the relative locations of very large objects. In such instances, the physical sizes of the objects can create problems in that portions of the large objects may mask or block the radio signals, thereby preventing them from reaching the receiver. The large objects can also be a source of multi-path interference, which again can make it difficult or impossible to accurately locate position.
Consequently, a need exists for a radio-based position location system that may be utilized in environments that are subject to interference (e.g., multi-path interference), and/or signal blocking due to the presence of large objects within the detection area.